1. Field of the Invention
The present invention relates to gyroscopes. More specifically, the present invention relates to fiber optic gyroscopes and associated signal processing circuitry.
2. Description of the Related Art
Gyroscopes are widely used to detect angular motion. Conventional mechanical gyroscopes were constructed with spinning masses which tended to resist translational and rotational motion of the host vehicle. Various sensors including potentiometers, differential inductive (e.g., microsyn), and differential capacitive methods have been used to sense the movement of the vehicle relative to the resistance of the gyroscope and provided a signal indicative of the angular motion.
Conventional gyroscopes required a spinning mass to detect angular motion. For certain applications, these masses, and their response to input motion about axes other than the intended input axis, imposed undesirable constraints on system design and/or performance. Accordingly, other technologies have been developed to detect acceleration. The fiber-optic gyroscope is one such alternative technology. The fiberoptic gyroscope employs a coil of optical fiber into which two beams of light are injected so that they travel in opposite directions therethrough. As the vehicle undergoes angular motion about the axis of the coil, light traveling in one direction will travel a shorter distance while light traveling in the opposite direction will travel a greater distance. Known as the xe2x80x9cSagnac Effectxe2x80x9d, the result is a phase shift that alters an interference pattern, created by recombining the two light beams. The intensity of the interference pattern is detected by a photodetector and converted to an electrical signal for subsequent processing.
Initially, the output of the fiber optic gyro interferometer was transformed to an electrical signal and processed with analog circuitry. Unfortunately, the associated analog demodulators were subject to various bias sources and electronic drift. Accordingly, numerous digital signal-processing techniques have been explored.
The classic xe2x80x9call-digitalxe2x80x9d signal processing technique for the interferometric fiber-optic gyroscope (FOG) was spelled out by H. C. Lefevre in xe2x80x9cHigh Dynamic Range Fiber Gyro with All-Digital Signal Processing,xe2x80x9d Fiber Optic and Laser Sensors VIII, SPIE Vol. 1367 (1990). One of the key aspects of this approach involved the replacement of an analog phase sensitive detector (PSD) that processes the output of the FOG photodetector with an analog to digital converter (ADC) and a digital PSD. This eliminated most of the sources of drift and bias.
However, since the Sagnac effect upon which the FOG is based is extremely weak, optical phase shifts of less than one microradian must be faithfully measured. This is done by scaling the inherent photon shot noise from the photodetector to be greater than one least significant bit (LSB) root mean square (rms) at the input to the ADC. This noise whitens the quantization and allows the high data rate interferometer output signal (100 kHz to 1 MHz typical) to be averaged deep into the LSB permitting the measurement of a signal with a very small amplitude.
Since Lefevre""s paper in 1990, many variations of signal gating, filtering and ADC sampling schemes have evolved within this framework. However, recent work has shown that small imperfections in quantization by the ADC (particularly INL and DNL errors) can cause large bias errors in the face of vibration, power supply drift or ripple on the power supply. These errors have caused the performance of the conventional fiber-optic gyroscope to be unacceptable for certain current applications. That is, since the averaging must go deeper than {fraction (1/1000)}th of the LSB, near perfection is required.
Consequently, there is a need in the art for a system and/or technique for improving the accuracy of fiber-optic gyro digital processing circuitry.
The need in the art is addressed by the present invention. In a most general description, the invention is a circuit for processing the output of a fiber-optic gyroscope. The inventive circuit includes a photodetector for detecting electromagnetic energy received from the fiber-optic gyroscope interferometer and providing an analog input signal in response thereto. An analog-to-digital converter is provided for processing the analog input signal and providing a digital signal in response thereto. A source is provided for injecting a dither signal between the photodetector and the analog-to-digital converter.
In the illustrative embodiment, the source supplies a triangle wave signal. The output of the photodetector is supplied to the summing input of a differential amplifier and the output of the source supplies the differencing input to the differential amplifier. The output of the analog-to-digital converter is provided to a digital signal processor as per usual practice.
Hence, the invention is a simple system and method for the addition of a dither signal to the input of the ADC. This signal has the effect of smoothing or averaging the non-linearities up through the digitization step and reduces the gyro drift caused by vibration and certain electrical disturbances. It has also been shown to reduce so called xe2x80x9ctogglingxe2x80x9d instability, warm-up drift, and other unidentified sources of bias instability in fiber-optic gyros.